Int. J. of Thermal & Environmental Engineering

Volume 4, No. 1 (2012) 47-54

* Corresponding author. Tel.: +971 2 810 9130

Fax: +971 2 698 8121; E-mail: ijanajreh@masdar.ac.ae

© 2011 International Association for Sharing Knowledge and Sustainability

DOI: 10.5383/ijtee.04.01.007

47

Thermodynamics Equilibrium Analysis within the Entrained Flow Gasifier Environment

Shabbar Syed

a, Isam Janajreh

a,*, Chaouki Ghenai

b

a Masdar Institute (MI), Abu Dhabi, 54224, United Arab Emirates

b Florida Atlantic University, Boca Raton, FL 33431, USA

Abstract

A thermodynamic equilibrium approach is used to calculate the maximum cold gasification efficiency (CGE) of

different feedstock within the entrained flow gasifier. The high temperature (1000 to 1500oK), the small feedstock size

(10μm to 0.1mm), and the well mixing environment make them most amenable for equilibrium analysis. Four different

feedstock, i.e. RTC coal, pine needles, ply-wood and lignite, are used for the gasification purpose. Using air as oxidizer

a parametric study is conducted to show the behavior of CGE and product gasification composition by increasing

oxidizer input. A gasification model is developed to calculate the seven unknowns in the gasification product. Using four

elemental mass balance and three equilibrium constant relations a mathematical code is developed that also incorporate

the solid un-burn carbon in product species. Energy equation is used to calculate the temperature of gasification using

iterative approach. Result of model shows that the maximum CGE is achieved when all the solid carbon is converted

into the carbon monoxide with nearly all hydrogen present in the feedstock converted into the hydrogen gas. Using this

result as baseline, a global model to estimate the maximum CGE is developed in reference to Van Krevelen diagram.

This model based on minimum requirement of air to convert all the carbon present in feedstock into carbon monoxide.

Result shows a growing trend of CGE with the increase O/C and H/C ratio of feedstock.

Keywords: Thermodynamics equilibrium, entrain flow gasifier, cold gasification efficiency

1. Introduction

Fossil energy resources that can be exploited economically are

believed to be finite. The developing economies are becoming

major consumers of these resources that also add pressure on

fossil fuel prices and supplies. The fear of depletion is a

recurring theme in the discussion of the future of energy.

Furthermore, the environmental concerns over the growing use

of fossil fuels, especially with respect to carbon dioxide (CO2)

emissions, have resulted in many calls for immediate action to

curtail the possibility of dangerous emission level that may

lead to global warming. Therefore, it should be a way to derive

energy to meet the future energy demand without harming

environment with the emissions associated with CO2, Nox Sox,

and CH4.

Thermo-chemical conversion (combustion, gasification, and

pyrolysis) is a core technology that can be utilized broadly to

generate energy without polluting the environment. Because of

its many applications, it is currently enjoying a considerable

renaissance [1]. Gasification is one of a thermo-chemical

conversion process through which clean fuels, chemicals and

efficiency augmentation can be produced. Gasification is

compatible with new applications in the area of biomass

conversion (when used with biomass, is carbon neutral); coal-

to-liquid; superior environmental performance especially with

regard to CO2 capture and sulfur removal; and the prospect for

substantial efficiency improvement when incorporated in

Integrated Gasification Combined Cycle plants (IGCC) for

electricity generation.

Gasification is a process in which a carbon containing

feedstock is thermo-chemically converted into syngas

comprising hydrogen (H2) and carbon monoxide (CO).

Gasification adds value to low or negative value feedstocks by

converting them into valuable fuel and/or chemical. Typical

feedstock used in the gasification process are coal, biomass and

industrial/municipal/agricultural as well as hazardous waste.

The production of syngas using gasification is a complex

process that depends on several factors including the

composition of feedstock, the gasifier conditions, temperature

and pressure, and the type and amount of oxidizer and

moderator (CO2 or Steam) [2]. The process of gasification

utilizes the partial combustion of feedstock by carefully

controlling the amount of oxidizer. The amount of oxidizer also

controls the temperature of the gasification process, which is

the most important parameter of gasifier, as reported by

Walawender [3].

Syed et al. / Int. J. of Thermal & Environmental Engineering, 4 (2012) 47-54

48

Gasification process is a series of exothermic and

endothermic process. Therefore a substantial amount of

thermal energy is required to sustain the process which

provided by either an external source of heat or by partial

combustion of feedstock [4]. During the partial combustion a

considerable amount (~25%) of feedstock is burned by the

gasification process providing energy to moisture release,

devolatilization, the endothermic Boudward and gas shift

reactions as well as the unrecovered sensible heat of the

product gas species and the ash. This amount is mainly depend

upon the composition and the heating value of the feedstock.

The gasification process mainly consists of initial preheating of

the feedstock that leads to drying. Further increase in

temperature causes pyrolysis during which light hydrocarbon

(volatiles) start evolving from the feedstock. Finally, the char

present in the feedstock either gasified or combusted leaving

behind ash. Depending upon the process parameter gasification

results mainly in formation of CO, H2, CH4 and CO2.

The efficiency of gasification process is the major concern that

decide the feasibly for the designed system. The efficiency is

measured in term of product syngas heating value to the

heating value of feedstock. This type of efficiency is termed as

cold gasification efficiency (CGE) as it ignores the resulted gas

and ash sensible heat and it is widely used to evaluate the

effectiveness of gasification process [5-8]. The only major

clean element present in the product gas is H2, because H2

combust to produce water. On other hand both CO and CH4 are

inclined to produce CO2 during the combustion and thus

producing environmental problems[9]. The methane gas can be

converted into H2 and CO at high temperature but the problem

still remain with CO as it combust to give CO2. Therefore, the

shift reaction is widely used to convert CO into H2 [10]. The

remaining CO2 is typically separated from the system prior to

utilization of the Gasification product into power generation,

i.e. Brayton Gas turbine cycle or fuel cell. In this way the clean

fuel is produce in term of H2 gas. So, the extent of H2 from the

gasification system is depending upon both the direct H2 gas

production from feedstock and indirect production via CO shift

reaction. In this way CO also contribute in the formation of

clean fuel and add the value in CGE.

Thermodynamic equilibrium modeling approach is widely used

to evaluate the performance of gasification system in terms

product gas composition and efficiency. Entrained flow

gasifiers are the most amenable for equilibrium modeling than

the fixed/moving and the bobbling bed gasifiers due to the

higher conversion temperature (~1300oK), smaller particle size

(m-0.1mm), higher mixing and heating rate environments

(~3000-10,000K/s) and the fastest conversion rate. Abuadala

[2] used equilibrium approach to improve the hydrogen

production by optimizing operating parameters of gasifier. The

study is mainly focused on the influence of gasification

temperature, feedstock feeding and moderator (steam) injection

on hydrogen yield and efficiency. Melgar [11] discuss the

thermo chemical process of a dawndraft biomass gasifier using

thermodynamic equilibrium model. The work includes a

parametric study of the influence of the gasifying relative

fuel/air ratio and the moisture contents of the biomass on the

characteristics of the process and the product gas composition.

Using rubber wood as feedstock with 14.7% moisture a cold

gasification efficiency of 80% was achieved. Sharma [12]

proposed a full equilibrium model of global reduction reactions

of a downdraft biomass gasifier in order to predict the accurate

distribution of various gas species, unconverted char and

reaction temperature. The maximum gasification efficiency of

the system was calculated 81% and the results were compared

with the experimental data. Zainal [13] develop a

thermodynamics equilibrium model to predict the gasification

process in a downdraft gasifier. The parametric study in terms

of initial moisture content in the feedstock and the temperature

in the gasification zone has been investigated. Finally, the

calorific value of product syngas was calculated to show its

declining trend with the increase in the moisture content in the

feedstock. Jarungthammachote [14] has developed a similar

type of equilibrium as Zainal [13] to calculate the composition

of product syngas for municipal solid waste using downdraft

gasifier. To enhance the performance of the model a further

modification was made by multiplying the equilibrium

constants with coefficients. Then the results were compared

with data reported by different experimental work. This model

works on auto-thermal gasification cycle in which temperature

of reaction zone was calculated using energy balance. This

approach provided a better understanding for calculating the

effect of moisture content and quantity of oxidizer on reaction

temperature. Mountouris [15] studied the thermodynamic

analysis of plasma gasification technology, which includes the

prediction of the product syngas, energy and exergy

calculations. This study also deals with the possibility of soot

formation, as a solid carbon byproduct. It also accounts for the

indirect energy provided to the gasifer from an additional out

source. The literature [13-15] investigated the thermodynamics

and kinetics involving the gasification process. It helps to

identify the nature of syngas at different operating conditions.

Also, different composition of feedstock can be handled.

In the view of above mention literature, a thermodynamic

equilibrium model is established in this study. The focus of this

study circles around the operating condition of an entrain flow

gasifier for which a maximum cold gas efficiency can be

achieved. This is accomplished by a parametric study of

varying oxidizer concentration inside the gasifier. The results

are present for the product gas compostion, char consumption,

gasification temperature and cold gasification efficiency. Based

on the result derived from the equilibrium analysis approach, a

case is developed of calculate the maximum cold gasification

efficiency using Van Krevelen diagram. Following

assumptions are made for this analysis:

Samples are taken as dry and ash free.

Steady state flow is considered inside the gasifier.

Potential and kinetic energies are neglected.

Only the major species are considered in the product gases,

i.e CO, CO2, H2, CH4 and N2.

The contribution of ash in energy equation is neglected.

Ideal gas behavior is considered for all gas phase.

The gasifer is considered to operate isothermally.

No heat losses are considered from the gasifier, i.e adiabatic

condition.

Infinite residence time is considered, so that the reaction

have ample amount of time to take place.

At each gasification step, all product gases are considered at

same temperature.

2. Feedstock Characterization

The baseline calculations are performed using four different

feedstocks. The feedstocks are selected to represent the

different regions of Van Krevelen diagram. As shown in Figure

1, the Van Krevelen diagram characterizes different type of

feedstocks according to their respective H/C and O/C ratio. The

highest H/C and O/C ratio represent the biomass and the lowest

represent the Anthracite, where coal, lignite and peat come in

between them.

Syed et al. / Int. J. of Thermal & Environmental Engineering, 4 (2012) 47-54

49

Figure 1: Van Krevelen Diagram [16]

Table 1 shows the ultimate analysis of dry ash free samples of

four different feedstock performed at Masdar Institute (MI) in

Waste to Energy laboratory using Flash (CHNS-O) analyzer.

The percent weight of carbon present in coal is highest as

compare to other three feedstocks. That shows the affinity of

coal to form more CO when air-gasified. On the other hand,

pine needles and ply-wood has high oxygen content that

reduces the amount of oxygen required for gasification. The

hydrogen contents of all feedstocks are within 1.65% to each

other. Also, the close resemblance in the ultimate analysis of

pine needles and ply-wood shows an expected same output

during the gasification process.

Table 1: Ultimate analysis of feedstock based on dry ash

free (DAF) basis

Feedstock

C

%Wt

H

%Wt

O

%Wt

N

%Wt

S

%Wt

RTC coal 82.17 5.60 8.60 2.50 1.13

Pine

needles 48.58 6.30 43.64 1.48 0.00

Ply-wood 49.59 6.28 43.74 0.39 0.00

Lignite[17] 66.03 4.65 25.64 2.07 1.62

The weight percent of each feedstock is used to calculate the

empirical formula by considering only carbon, hydrogen,

oxygen and nitrogen. Table 2 shows the empirical formula

corresponding to each feedstock. These formulas are calculated

on molar bases by normalizing through a single atom of

carbon. The higher heating value (HHV), as shown in Table 2,

are calculated using the following relation [18],

where C, H and O are the corresponding weight percentage of

carbon, hydrogen and oxygen present in each feedstock. The

results show that the coal has the highest HHV while pine

needles and ply-wood have their HHV near to each other.

Table 2: Empirical formula and heating value (HHV) of

feedstock

Feedstock Empirical formula HHV KJ/Kmole

HHV MJ/Kg

RTC coal CH0.7946O0.0670N0.0260 502928 34.38 Pine needles CH1.5550O0.6736N0.0261 489784 19.83 Ply-wood CH1.5196O0.6615N0.0067 487566 20.14

Lignite CH0.8450O0.2912N0.0268 469939 26.28

3. Gasification Model

The Global reaction of air gasification can be written as,

where x, y and z are number of atoms of hydrogen, oxygen and

nitrogen based on a single atom of carbon. The right hand side

of Equation (2) contains a single kmole of feedstock and

known kmole of air (m). The left hand side of the Equation (2)

composed of six unknowns, where, x1, x2, x3, x4, x5 and x6 are

the kmole of each corresponding species. Nitrogen (N2) is not

taken as an unknown because it is just a sum from the

feedstock and oxidizer. Also, N2 is taken as un-reacted specie,

so it goes in the reaction and comes out un-reacted. The one

thing worth to mention at this point, that all the quantities x, y,

z, x1, x2, x3, x4, x5 and x6 are based on single atom of carbon

present in feedstock.

3.1. Species mass balance

To calculate the value of six unknown, six equations are

required. Three equations can be derived by the conservation of

elemental mass present in the reactants and products.

Followings are the mass balance of each element present in the

Equation (2),

carbon balance

hydrogen balance

oxygen balance

The nitrogen moles can directly be calculated by using of

nitrogen present in feedstock and air, i.e.

. This

value can be used to calculate the overall mole fraction, mass

fraction of syngas and energy balance.

3.2. Equilibrium constant formulation

Now three more equations are required that solved together

with equation (3), (4) and (5) to calculate the equilibrium

composition of product syngas. These equations are derived

using the knowledge of equilibrium constants of elementary

reactions. Higman [19] has described the elementary reactions

as the intermediate reactions that leads to complete

gasification. Solid carbon elementary reactions involving

carbon (C), CO, CO2, H2, water (H2O) and CH4 are necessary

to describe the overall gasification process. The reactions are as

follows,

boudouard reaction

CO shift reaction

Ato

mic

H/C

Rati

o

Atomic O/C Ratio

Coal

Lignite

Peat

Biomass

Anthracite

0 0.2 0.4 0.6 0.8

Pine needle & ply-wood

Lignite

RTC Coal

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Syed et al. / Int. J. of Thermal & Environmental Engineering, 4 (2012) 47-54

50

methanation reaction

Therefore, the equilibrium constant is written for equation (6),

(7) and (8) that enables to complete the six equation required to

calculate the equilibrium composition of product syngas.

Equilibrium constant for boudouard reaction

Equilibrium constant for CO shift reaction

Equilibrium constant for methanation reaction

where xtotal is the total number of moles present in the product

of Equation (2). That brings a new equation in the system and

it is the total sum of moles of product species as follows,

The char (x6) represent the solid un- burnt carbon present in the

product. Therefore charis excluded from the equation (12), that

represent the sum of moles of gaseous phase.

The value of equilibrium constant is found out at constant

temperature and pressure using the standard state Gibbs

function of change( [21].

where R is the universal gas constant

and

T is the absolute temperature in Kelvin. The value of is

calculated by taken the species present in the relevant

elementary reactions. Therefore the value of for

boudouard, CO shift and methanation reaction is formulated,

for boudouard reaction

for CO shift reaction

similarly, for methanation reaction

where represents Gibbs function of the pure species

at standard state pressure (P=1 atm) at constant temperature.

The value of at constant temperature can be

expressed by the following empirical formula [20],

The value of coefficients a, b, c, d, e, f, g and enthalpy of

formation is presented in Table 3. The value of

for C

and H2 is zero over the temperature range. Therefore, the

contribution and

in Equation (14), (15) and (16)

is neglected.

Summarizing, the value of from equation (17) is

calculated for each species at corresponding temperature. Then

this value is used in Equation (14), (15) and (16) to compute

the value of for boudouard, CO shift and methanation

reaction. After that, the value of K1, K2 and K3 are calculated

using Equation (13). The value of equilibrium constants are

used in Equation (9), (10) and (11) so that the only unknowns

are the kmole of species. Finally, Equation (3), (4), (5), (9),

(10) and (11) are solved simultaneously to find the equilibrium

mole fraction of species.

3.3. Energy equation

In order to calculate the adiabatic temperature of gasification

the energy balance between reactant and product is used,

where xi is the number of moles of reactants,

is the

enthalpy of formation of reactants , is the number of moles of

product species, is the enthalpy of formation of product

species and is the sensible enthalpy of product. The above

equation can be expressed to the following expanded form

when applied to equation (2)

Table 3: The value of and coefficients a, b, c, d, e, f, g in equation (17) for different species [20]

Species hf (KJ/Kmole) a b c d e f g

CO -110.5 5.619x103 -1.190x105 6.383x109 -1.846x1012 -4.891x102 8.684x101 -6.131x102

CO2 -393.5 -1.949x102 3.122x105 -2.448x108 6.946x1012 -4.891x102 5.27 -1.207x101

H2O -241.8 -8.950x103 -3.672x106 5.209x109 -1.478x1012 0 2.868 -1.722x102

CH4 -74.8 -4.620x102 1.130x105 1.319x108 -6.647x1012 -4.981x102 1.411x101 -2.234x101

Syed et al. / Int. J. of Thermal & Environmental Engineering, 4 (2012) 47-54

51

where Cp is the specific heat at constant pressure and ΔT is the

change in temperature with respect to reference temperature

(T1=298K). The enthalpy of formation for O2 and N2 are

zero at the reference state therefore the last two terms on left

hand side of Equation (19) are eliminated. The enthalpy of

formation of feedstock is calculated using the method defined

by Zainal et al. [13]. In the calculation the presence of nitrogen

in the feedstock is neglected as its contribution is small. The

general formulation follows as,

The above reaction is a hypothetical reaction showing the

formation of feedstock. The near to realistic mechanism that

contribute the formation feedstock follows as,

The value of heat of combustion in Equation (22) is based on

the heating value of the feedstock. If the lower heating value

(LHV) is used then it is necessary to use the heat of formation

of water vapor. In the present calculations the HHV of the

feedstock is used, therefore the heat of formation of liquid

water is used. The resultant reaction attributed to HHV of

feedstock and is written as,

Adding Equation (20), (21) and (22) yields,

The above formula is used to calculate the enthalpy of

formation of feedstock. The enthalpy of formation of species in

Equation (19) is used from Table 4.

The average specific heat over the temperature range used in

Equation (19) is calculated by the following the formula of

[22],

(25)

where T is the adiabatic gasification temperature acts like the

only unknown in Equation (19), T1 is the reference temperature

(298 K), R is the universal gas constant, Taverage is the

arithmetic mean of T and T1 i.e.

and A, B, C and D are

the coefficients that are available in Table 5. The Cp for carbon

is calculated from the following polynomial developed by

using the NIST-JANAF table [23],

Table 4: Enthalpy of formation of species [22]

Species hf(KJ/Kmole)

Carbon dioxide (CO2) 394359

Carbon monoxide (CO) 137169

Methane (CH4) 50460

Water (liquid) (H2O) 237129

Water (vapor) (H2O) 228572

Hydrogen (H2) 0

Oxygen (O2) 0

Nitrogen (N2) 0

Carbon (C) 0

Table 5: Constant used for specific heat capacity in

Equation (25) [13]

Species Tmax A 103B 106C 10-5D

Methane 1500 1.702 9.081 2.164 0

Hydrogen 3000 3.249 0.422 0 0.083

Carbon

monoxide

2500 3.376 0.557 0 0.031

Carbon

dioxide

2000 5.457 1.047 0 1.157

Nitrogen 2000 3.28 0.593 0 0.04

Water 2000 1.771 0.771 0 0.121

Summarizing energy equation,

1. The value of m, x1, x2, x4, x5 and x6 are known from

previous step.

2. The enthalpy of formation of feedstock is calculated using

Equation (24).

3. The enthalpy of formation of other species is taken from

Table 4.

4. Specific heat at constant pressure is calculated using

Equation (25) and (26).

5. Finally, the Equation (19) is solved for only unknown

value of T, the adiabatic gasification temperature.

3.4. Calculation Procedure

At the start an initial value of T is assumed i.e. T=300 K. At

this value the equilibrium moles of product syngas is computed

using Equation (3), (4), (5), (9), (10) and (11). After that, the

values of moles are substituted in energy Equation (19) to

Syed et al. / Int. J. of Thermal & Environmental Engineering, 4 (2012) 47-54

52

calculate the actual value of adiabatic gasification temperature

(T). Then the calculated value of T is again used to calculate

equilibrium moles of species. This process is repeated until the

value of T become stable and that gives the actual adiabatic

gasification temperature and mole of product species.

4. Result and Discussion

The model is run for all four feedstocks with varying air

supply. The increasing amount of air causes to rise up the

temperature to convert all the char, yet burning more of the

feedstock. At every step the cold gas efficiency (CGE) is

calculated using the following formula,

The product gases that contribute in the higher heating value

are carbon monoxide, hydrogen and methane. The inclusion of

HHV of product gases gives the updated formula of CGE as,

Now, using the gasification model the results are presented

showing the product gas composition, temperature and CGE

through parametric study of oxidizer input. Figure 2 shows the

result of equilibrium analysis for RTC coal. It shows that the

increase in the amount of air tends to raise the temperature of

the product. The rise in temperature causes to produce more

carbon monoxide in initial phase that directly increase the

CGE. At 1024 K the CGE reaches to its maximum value of

68.5%. It is evident from the graph that CGE act like a normal

distribution, i.e having a maximum value at certain point that

tends to decrease at both ends. At maximum CGE, the kilo

mole of CO, CO2, H2 and H2O are 0.91, 0.09, 0.37 and 0.03,

respectively. Further increase in air supply tends to convert CO

into CO2 and H2 into H2O,that brings the CGE to lower value.

The results are based on kilo mole of product per kilo mole of

the feedstock, that shown a very small quantity of methane

(nearly zero) in the results. Due to high HHV of methane,

small quantity can lead to a significant change in the CGE, but

the result of analysis shows that at point of maximum CGE the

quantity of methane is of order minus four. It makes the

contribution of methane almost negligible to account for

maximum CGE.

Figure 2 show the results of equilibrium analysis for pine

needles. The overall trend of the graph is similar to Figure 3.

The maximum CGE of 76% is achieved at 978oK. This

significantly increased value of maximum CGE as compare to

RTC coal is achieved because of increased hydrogen and

oxygen content in the pine needle. The rich hydrogen content

in the pine needle enable to produce more hydrogen gas in

product mixture and while the oxygen present in the mixture

tends to react with solid carbon to make carbon monoxide. The

high content of oxygen present in pine needles also enables to

utilize lesser amount of air to achieve the desire temperature.

Figure 4 shows the equilibrium analysis of ply-wood. The

maximum CGE of 76.5% is achieved at 980o K. These values

are in close proximity with the results achieved for pine needle.

The difference in the temperature is by the virtue of small

change in composition and HHV. Similarly, Figure 5 shows

the maximum CGE of 74% for lignite at 1134 oK temperature.

The results of all four feedstock shows that the maximum CGE

is achieved when all the carbon present in feedstock is

converted into CO or CO2 while hydrogen content of feedstock

transformed in to H2 gas or H2O. At maximum CGE, on

average 15% of carbon is converted into CO2 and rest form

CO. Similarly, 10% of hydrogen present in the feedstock

converted in H2O and rest goes to form H2. By using this

conclusion, the maximum CGE of any feedstock present on

Van Krevelen diagram can be calculated by using following

reaction:

Figure 2: Equilibrium analysis for RTC coal showing CGE,

Temperature and product gas composition

Figure 3: Equilibrium analysis for Pine wood showing

CGE, Temperature and product gas composition

Syed et al. / Int. J. of Thermal & Environmental Engineering, 4 (2012) 47-54

53

Figure 4: Equilibrium analysis for Ply-wood showing CGE,

Temperature and product gas composition

Figure 5: Equilibrium analysis for Lignite showing CGE,

Temperature and product gas composition

Figure 6: Straight line approximation for Van Krevelen

diagram

As the O/C ratio on Van Krevelen diagram remains less than

one, therefore the equation (29) remains valid for the entire

region of the diagram. To demonstrate the process, a straight

line is created on Van Krevelen diagram that passes through all

major four zones, as shown in Figure 6. The equation (30)

shows the equation of the line derived from Van Krevelen

diagram. Only thing that account to develop the polynomial is

to stay in the region inside the shaded area of diagram. The

equation (30) provides the composition x of feedstock by

specifying the amount of y. Although, a high order polynomial

can be developed that account for all zones of the diagram, but

for illustration, only a simplest line approach is used.

The equation (1) is based on the weight percent of carbon,

hydrogen and oxygen but it can be modified to produce the

desired result of HHV by providing the value of x and y, as

follows,

The value of x and y in equation (30) and (31) are same as

specified for the feedstock in equation (2). These values are

actually H/C and O/C ratio used in Van Krevelen diagram.

Now, to calculate the maximum CGE the value of x is

calculated by increasing value of y. Once, the value of x and y

are known then equation (31) is used to calculate the HHV of

the corresponding feedstock. The maximum kmoles of carbon

monoxide and hydrogen is calculated using equation (29).

After that, equation (28) is used to calculate the maximum

CGE for the corresponding feedstock. The results are tabulated

in Table 6 over the feedstock range on Van Krevelen diagram.

The result shows an increasing trend of CGE by increasing the

amount of x and y. The mole of carbon monoxide is held 0.85

because it is assumed that 85% of carbon present in feedstock

is converted into CO at CGE.. Also, the excess amount of

oxygen need to form a single mole of carbon monoxide is

provided from the air input from outside the system. The

formation of hydrogen is only dependent on the value of x,

which is the only source of hydrogen in the system. So, as the

value of x increase the number of moles of hydrogen increase.

Table 6 shows a growing trend of CGE with increasing value

of x and y. Practically, it is nearly impossible to achieve these

high value of CGE. This high efficiency is because the usage

of dry and ash free samples for the analysis. If the moisture and

ash is considered for the analysis then they lower the

temperature of the gasification process and eventually lower

the system efficiency. But one thing is obvious from the Table

6 that increases in the value of x and y will also increase the

CGE.

Table 6: Maximum CGE by complete conversion of carbon

into carbon monoxide

#

y

x

HHV

(kJ/kmole)

CO

(kmole)

H2

(kmole)

CGE

(%)

1 0.10 0.76 491672.04 0.85 0.34 68.74

2 0.20 0.87 488105.59 0.85 0.39 72.15

3 0.30 0.98 484539.14 0.85 0.44 75.61

4 0.40 1.09 480972.69 0.85 0.49 79.12

5 0.50 1.20 477406.24 0.85 0.54 82.69

6 0.60 1.32 473839.78 0.85 0.59 86.30

7 0.70 1.43 470273.33 0.85 0.64 89.98

8 0.80 1.54 466706.88 0.85 0.69 93.71

5. Conclusion

A thermodynamics equilibrium approach is used to calculate

the maximum cold gasification efficiency of the gasification

process. Air is used as oxidizer to gasify four different

feedstock. Using the result of ultimate analysis of feedstock the

empirical formula and heating values are calculated. The

empirical formula is used to build the global gasification

reaction that shows seven species (including solid carbon) as

the product of gasification. To calculate the product

composition seven equations are used that represented the four

elemental balance equations and the three equilibrium

constants. The gasification temperature is calculated using the

energy equation following an iterative process. The result

shows that, on dry and ash free basis, the maximum

gasification efficiency of 68.5%, 76.0%, 76.5% and 74.0% can

be achieved for RTC coal, pine needle, ply-wood and lignite,

respectively. The trend of results shows that the maximum

CGE is achieved when most ofthe solid carbon present in the

feedstock is converted into carbon monoxide. Using the

maximum potential of carbon monoxide, an idealized approach

is used to calculate the CGE for Van Krevelen diagram. It

shows increasing O/C and H/C ratio directly affects the CGE.

The increase in the value of both ratios gives a raise to the

CGE.

Ato

mic

H/C

Rati

o

Atomic O/C Ratio

Coal

Lignite

Peat

Biomass

Anthracite

0 0.2 0.4 0.6 0.8

Pine needle & ply-wood

Lignite

RTC Coal

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Syed et al. / Int. J. of Thermal & Environmental Engineering, 4 (2012) 47-54

54

References

[1]. DOE, Gasification World Database 2007 (Current

Industry Status Report)”, Office of Fossil Energy

National Energy Technology Laboratory (NETL) 2007.

[2]. Abuadala, A., I. Dincer, and G. Naterer, Exergy analysis

of hydrogen production from biomass gasification.

International Journal of Hydrogen Energy, 2010. 35(10):

p. 4981-4990. doi:10.1016/j.ijhydene.2009.08.025

[3]. Walawender, W., D. Hoveland, and L. Fan, Steam

gasification of pure cellulose. 1. Uniform temperature

profile. Industrial & Engineering Chemistry Process

Design and Development, 1985. 24(3): p. 813-817. doi:10.1021/i200030a048

[4]. Rezaiyan, J. and N. Cheremisinoff, Gasification

technologies: A primer for engineers and scientists.

2005: CRC.

[5]. Nakamura, A., et al., Gasification of catalyst-suspended

chicken manure in supercritical water. Journal of

Chemical Engineering of Japan, 2008. 41(5): p. 433-440. doi:10.1252/jcej.07WE289

[6]. Xiang, W., S. Wang, and T. Di, Investigation of

gasification chemical looping combustion combined

cycle performance. Energy & Fuels, 2008. 22(2): p. 961-

966. doi:10.1021/ef7007002

[7]. Pathak, B., et al., Performance evaluation of an

agricultural residue-based modular throat-type down-

draft gasifier for thermal application. Biomass and

bioenergy, 2008. 32(1): p. 72-77. doi:10.1016/j.biombioe.2007.06.006

[8]. Sharma, A., Modeling and simulation of a downdraft

biomass gasifier 1. Model development and validation.

Energy Conversion and Management, 2010.

[9]. Tol, R., The marginal damage costs of carbon dioxide

emissions: an assessment of the uncertainties. Energy

policy, 2005. 33(16): p. 2064-2074. doi:10.1016/j.enpol.2004.04.002

[10]. Chen, W., T. Hsieh, and T. Jiang, An experimental study

on carbon monoxide conversion and hydrogen

generation from water gas shift reaction. Energy

Conversion and Management, 2008. 49(10): p. 2801-

2808. doi:10.1016/j.enconman.2008.03.020

[11]. Melgar, A., et al., Thermochemical equilibrium

modelling of a gasifying process. Energy Conversion

and Management, 2007. 48(1): p. 59-67. doi:10.1016/j.enconman.2006.05.004

[12]. Sharma, A., Equilibrium modeling of global reduction

reactions for a downdraft (biomass) gasifier. Energy

Conversion and Management, 2008. 49(4): p. 832-842. doi:10.1016/j.enconman.2007.06.025

[13]. Zainal, Z., et al., Prediction of performance of a

downdraft gasifier using equilibrium modeling for

different biomass materials. Energy Conversion and

Management, 2001. 42(12): p. 1499-1515. doi:10.1016/S0196-8904(00)00078-9

[14]. Jarungthammachote, S. and A. Dutta, Thermodynamic

equilibrium model and second law analysis of a

downdraft waste gasifier. Energy, 2007. 32(9): p. 1660-

1669. doi:10.1016/j.energy.2007.01.010

[15]. Mountouris, A., E. Voutsas, and D. Tassios, Solid waste

plasma gasification: Equilibrium model development

and exergy analysis. Energy Conversion and

Management, 2006. 47(13-14): p. 1723-1737. doi:10.1016/j.enconman.2005.10.015

[16]. Ptasinski, K., M. Prins, and A. Pierik, Exergetic

evaluation of biomass gasification. Energy, 2007. 32(4):

p. 568-574. doi:10.1016/j.energy.2006.06.024

[17]. Agraniotis, M., et al., Experimental investigation on the

combustion behaviour of pre-dried Greek lignite. Fuel

Processing Technology, 2009. 90(9): p. 1071-1079. doi:10.1016/j.fuproc.2009.04.015

[18]. Prins, M., K. Ptasinski, and F. Janssen, From coal to

biomass gasification: Comparison of thermodynamic

efficiency. Energy, 2007. 32(7): p. 1248-1259. doi:10.1016/j.energy.2006.07.017

[19]. Higman, C. and M. Van Der Burgt, Gasification. 2003:

Gulf Professional Publishing.

[20]. Probstein, R. and R. Hicks, Synthetic fuels. 1990.

[21]. Stephen, R., Turns. An introduction to combustion:

concepts and applications. Mechanical Engineering

Series. McGraw Hill, 2000.

[22]. Green, D., Perry's chemical engineers' handbook. 1984:

McGraw-Hill New York.

[23]. Chase, M., N.I.o. Standards, and Technology, NIST-

JANAF thermochemical tables. 1998.